Gear Oil vs Engine Oil for Transmission

[annihil8ted]

Hello! I have a 98 Sahara so the AX-15 manual transmission.

I understand that the go to recommendation is MT90 or MTL for the transmission fluid but I've also read a good synthetic 10w-30 will work as well. Is this accurate?

I'm wondering are there any benefits lost switching to engine oil? I ask because I'd like to run engine oil for maybe 500 miles to flush out the transmission but just incase I don't get back to it, running it long term. Another thing I'm curious about is if there's enough of the necessary additives in engine oil compared to gear oil. Lastly, cost: MT90 was 70 for a gallon when I checked today and a good 10w-30 near me is less than 20.

Thanks!

 

[Jerry Bransford]

Yes that is accurate, Aisin officially added a synthetic 10W-30 to its recommended GL-4 75W-90 gear lube (like Redline MT-90 is) since a GL-4 75W-90 is so hard to find. Because they are measured on two different viscosity scales the actual viscosity of a 10W-30 engine oil is roughly the same as a 75W-90 gear lube.

 

[JKP]

I've run the factory fill, MT90, syn 75W-90 that claimed it was yellow metal safe, and now 0W-40 syn in my YJ AX-15. Only difference I could tell is the synthetics weren't stiff on cold mornings like the non syn is. About 300K on it so far.

 

[annihil8ted]

Thank you! That gives me a good feeling as often in the mornings, it feels like the system is being held back, almost as if the fluids are too thick prior to warming up? Shifting into first or second dropps RPM severely and adding gas causes shudder. They go away after a while of driving (which I assume is the fluid warming up). Hopefully switching to synthetic will help with that.

How about in terms of additives? Will running engine oil make me miss out on certain gear oil additive benefits?

 

[Jerry Bransford]

Thank you! That gives me a good feeling as often in the mornings, it feels like the system is being held back, almost as if the fluids are too thick prior to warming up? Shifting into first or second dropps RPM severely and adding gas causes shudder. They go away after a while of driving (which I assume is the fluid warming up). Hopefully switching to synthetic will help with that.

How about in terms of additives? Will running engine oil make me miss out on certain gear oil additive benefits?

Synthetics flow easily and lubricate well in cold temperatures, and the Redline MT-90 and synthetic 10W-30 are both synthetics. You won't be missing out on any additives by running the synthetic 10W-30 engine oil. Or you could just order 3 quarts of the Redline MT-90 on Amazon.

 

[JKP]

The main thing that's missing is the sulphur compounds for extreme pressure which the trans doesn't need and will eat the yellow metals in the trans.

If you go to Summit Racing, etc, they now have a selection of GL4 gear oils which you could use in the trans at somewhat reasonable prices. However, nothing is going to be cheaper than syn motor oil. 5 qts of M1 0W-40 is around 25 bucks at Walmart. The house brand is probably even cheaper.

Never use motor oil or GL4 in the diffs.

 

[MGB]

I’m thankful this topic came up again because it allows me to ask a question. 8,000 miles ago ( at 186,000) I had a new clutch assembly, replaced fly wheel, replaced clutch slave and master cylinder all done by a shop.

My daughter drives this jeep mostly and has occasional problems with the shifting feeling stiff and getting it into first after stops. The latest was after driving it for about 45 minutes in the recent Texas heat. She says she puts the clutch all the way to the firewall and even when I drive it it seems like the clutch travel must be 80% to the floor before it does anything. I start to wonder about all the fluids I replaced 20,000 miles ago and if I need to replace the transmission fluid again, this soon. I used Redline MT-90 GL4 (had to double check I used the right stuff).

By the way, thanks for all the great help from the moderators/experts. I rebuilt my double Cardin joints about six months ago after discovering the angry sparrow situation, listening to guys like you all.

 

[JKP]

That sounds like the clutch is not fully releasing.

 

[Jerry Bransford]

The main thing that's missing is the sulphur compounds for extreme pressure which the trans doesn't need and will eat the yellow metals in the trans.

Just so there's no confusion, there are no sulfur compounds/additives used in our manual transmissions. Those are strictly for GL-5 gear lubes needed by our axles.

 

[SvtLdr]

Hello! I have a 98 Sahara so the AX-15 manual transmission.

I understand that the go to recommendation is MT90 or MTL for the transmission fluid but I've also read a good synthetic 10w-30 will work as well. Is this accurate?

I'm wondering are there any benefits lost switching to engine oil? I ask because I'd like to run engine oil for maybe 500 miles to flush out the transmission but just incase I don't get back to it, running it long term. Another thing I'm curious about is if there's enough of the necessary additives in engine oil compared to gear oil. Lastly, cost: MT90 was 70 for a gallon when I checked today and a good 10w-30 near me is less than 20.

Thanks!

I've been using Mobil 1 10W-30 in my AX-15 for about 10 years now. I change it every other year, so maybe every 20k miles? No issues whatsoever.

 

[Ticks]

Shifting into first or second dropps RPM severely and adding gas causes shudder.

Sounds like your clutch isn't disengaging as intended. Changing the transmission fluid isn't going to make it better.

 

[freeskier93]

Personally I like Redline MT-90/MTL more, I thought 10w-30 felt really notchy.

 

[annihil8ted]

Thanks everyone!

Sounds like your clutch isn't disengaging as intended. Changing the transmission fluid isn't going to make it better.

Disengaging or engaging? Waiting for the clutch to "catch" seems to drop the engine RPMs really low and when I try to maintain the RPMs, I get shuddering before finally mating. Worse in second gear from a stop, first is more manageable

 

[MGB]

That sounds like the clutch is not fully releasing.

Yeah. I was hoping for some common fix for this so that it would act like it did with less miles.

 

[JKP]

Yeah. I was hoping for some common fix for this so that it would act like it did with less miles.

My 92 YJ AX-15 does this sometimes, other times not. About 300K on it. In the YJ forums we have not really found a reason, just several theories. Worn input shaft bearing, worn thrust bearing in engine are the ones I remember. Seems to be a high mileage issue though. I would lean toward the input shaft bearing in the trans.

 

[MGB]

My 92 YJ AX-15 does this sometimes, other times not. About 300K on it. In the YJ forums we have not really found a reason, just several theories. Worn input shaft bearing, worn thrust bearing in engine are the ones I remember. Seems to be a high mileage issue though. I would lean toward the input shaft bearing in the trans.

Thanks. I’ll make a note of that and in the mean time check I’m up to date with AAA for my daughter. I just replaced the heater core. I don’t know why this TJ hates me.

 

[JKP]

If it is the same issue, what is happening is the input shaft is turning when it shouldn't be. This makes it hard/impossible to get into first and will grind when trying to go into reverse. If you can't get it in gear, turn the engine off and it should drop right in. Then start it up and drive away.

 

[MGB]

If it is the same issue, what is happening is the input shaft is turning when it shouldn't be. This makes it hard/impossible to get into first and will grind when trying to go into reverse. If you can't get it in gear, turn the engine off and it should drop right in. Then start it up and drive away.

I will definitely pass that on. She usually gets it going just about 30 seconds after she calls me all distressed.

 

[annihil8ted]

Another quick question on transmission service; how critical is it to have the transmission (and transfer case) flat before servicing?

I'm in the middle of a transmission mount swap so I figured this would be a good time to drain both the transfer case and the transmission. The problem though is I've parked the TJ up ramps so only the front is raised by about a foot.

Things I forseee are not fully draining (for the transmission as the drain and fill are high points facing front) and not fully filling (for the transfer case which is facing back).

Is it worth the effort to try and raise the rear?

 

[JKP]

It needs to be level to get the correct amount in when filling them. I wouldn't really about the little bit left when draining the trans unless you had water intrusion.